Universal joints are used in mechanical applications to transmit torque between components where there can be misalignment of rotating parts. In a drilling operation, a drill bit is mounted to the end of a drill string. The drill bit is rotated from the top of the string or by a motor at the bottom of the string, or both, to advance the borehole. Universal joints are included in the drill string to accommodate rotational eccentricity. The eccentric rotation is converted into axial rotation in order for the drill bit to advance the borehole efficiently. Eccentricity can be initiated by a motor in the drive assembly that rotates the drill bit or by steering of the bit to change direction of the borehole.
FIG. 1 is a schematic representation of a drilling operation 2. In conventional drilling operations a drill bit 8 is mounted on the end of a drill string 6 comprising drill pipe and drill collars. The pipe sections of the drill string are threaded together at their ends to create a pipe of sufficient length to reach the bottom of the wellbore 4. A pump circulates drilling fluid through the drill pipe and out of the drill bit flushing rock cuttings from the bit and transporting them back up the wellbore. Additional tools 10 can be included in the drill string such as motors and vibrators.
The components of the drill string including the universal joint are subjected to extreme torque forces, elevated operating temperatures and abrasive drilling fluids, all of which can have an adverse effect on the operational life of drill string components. Constant relative movement of the components of a universal joint during operations, together with abrasive drilling mud, causes abrasion and erosion of mating components. Due to the constant relative movement of the components and aggressive downhole environment, they do not readily lend themselves to conventional sealing arrangements. Replacement of the joint or its components requires removal of the drill string from the borehole and downtime for the operation which increases operational expenses substantially.